A MIMO transmission is used to improve a transmission rate with limited frequency resources, and different pieces of data are transmitted from a plurality of correlated antennas, there by performing spatial multiplexing. Thus, a transmission rate can be improved without increasing the frequency bands. The MIMO technology is expected to be applied to a next-generation mobile radio communication system mainly for a high-speed data communication such as LTE (long term evolution), WiMax (worldwide interoperability for microwave access), etc.
FIG. 1 illustrates an example of the configuration of the conventional MIMO transmission system (2×2 MIMO). A cell 103 corresponding to a base station apparatus 101 is formed by three sectors 1 through 3, and the base station apparatus 101 has the configuration as illustrated in FIG. 2.
The base station apparatus illustrated in FIG. 2 includes antennas 111 through 116, transmission/reception units (TRXs) 201 through 206, baseband processing units (BBs) 211 through 213, and an interface (INT) 221. Among them, the transmission/reception units 201 through 206 and the baseband processing units 211 through 213 are connected to one another via a bus 231. In place of the bus 231, a mesh connection can be adopted. The interface 221 communicates with a base station control device through a cable transmission line.
Each branch of sectors 1 through 3 is configured by the following combination of an antenna and a transmission/reception unit.                1. branch Br1 of sector 1: antenna 111 and transmission/reception unit 201        2. branch Br2 of sector 1: antenna 112 and transmission/reception unit 202        3. branch Br1 of sector 2: antenna 113 and transmission/reception unit 203        4. branch Br2 of sector 2: antenna 114 and transmission/reception unit 204        5. branch Br1 of sector 3: antenna 115 and transmission/reception unit 205        6. branch Br2 of sector 3: antenna 116 and transmission/reception unit 206        
Since the MIMO transmission can be performed in an area where a plurality of antennas can receive data, the following conditions are considered to prevent the MIMO transmission between the base station apparatus 101 and a mobile station 102 in the cell 103.
(1) Cell Edge
The correlation of antennas cannot be recognized when the mobile station 102 is apart from the base station apparatus 101.
(2) Sector Boundary
The MIMO transmission cannot be selected by handover when the mobile station 102 is approaching the boundary of sectors.
The mobile station 102 normally performs the MIMO transmission by selecting a plurality of antennas in good reception statuses. However, when the selected antennas belong to respective different sectors, the MIMO transmission is not selected, and a fast cell selection (FCS) or a soft handover (SHO) is selected because a control target in the conventional scheduling is assigned to a sector and the MIMO transmission over sectors is not defined.
For example, when the mobile station 102 is located around the center of the sector 1 the signal-to-interference ratios (SIRs) of the branches Br1 and Br2 of the sector 1 are sufficiently large as illustrated for the case C1 in FIG. 3, and the reception status is good. Therefore, a 2×2 MIMO transmission is performed using the antennas 111 and 112.
Next, when the mobile station 102 moves in the vicinity of the boundary between the sectors 1 and 2, the SIRs of the branch Br2 of the sector 1 and the branch Br1 of the sector 2 are large, but the SIRs of the branch Br1 of the sector 1 and the branch Br2 of the sector 2 are small as illustrated for the case C2. Therefore, it is hard to perform the 2×2 MIMO transmission, and the FCS or the SHO is normally applied.
Next, when the mobile station 102 moves in the vicinity of the center of the sector 2, the SIRs of the branches Br1 and Br2 of the sector 2 are large as illustrated for the case C3, and the 2×2 MIMO transmission is performed using the antennas 113 and 114.
Thus, when the mobile station 102 moves from the sector 1 to the sector 2, it is necessary to switch the connection of the user data by handover. As illustrated in FIG. 4, when the mobile station 102 exists in the sector 1, the baseband processing unit 211 is connected to the transmission/reception units 201 and 202, and the signal processing for the user data 401 is performed.
If the transmission/reception units 201 through 204 are connected to the baseband processing unit 211 through the bus 231 or a mesh as illustrated in FIG. 2, the connection is switched as illustrated in FIG. 5 when the mobile station 102 moves to the sector 2. In this case, the baseband processing unit 211 is connected to the transmission/reception units 203 and 204.
On the other hand, as designed for the HSDPA (high speed downlink packet access), if the baseband processing units 211 and 212 are respectively assigned to the sectors 1 and 2, and the mobile station 102 moves to the sector 2, then the user data 401 is moved to the baseband processing unit 212 as illustrated in FIG. 6. In this case, the baseband processing unit 212 is connected to the transmission/reception units 203 and 204.
While the mobile station 102 exists around the boundary of the sectors land 2, the FCS is applied as illustrated in FIG. 7. In this case, the baseband processing unit 211 is connected to the transmission/reception units 201 through 204, and the same data is transmitted to the sectors 1 and 2.
As described above, while the mobile station 102 exists around the center of the sector 1 or 2, user data can be transmitted at a high speed in the 2×2 MIMO transmission. However, while the mobile station 102 exists around the boundary of the sectors 1 and 2, control is passed to the FCS/SHO. Therefore, the transmission rate is decreased, and the maximum transmission rate may not be attained.
The following patent document 1 relates to a communication system capable of switching selection diversity/MIMO transmission using a plurality of antennas, and the patent document 2 relates to a system of dividing base station antennas into a plurality of array groups and controlling a directional beam in each array group. The patent document 3 relates to a system of transmitting data in a multiple diversity transmission mode.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-333443
Patent Document 2: Japanese Laid-open Patent Publication No. 2003-338781
Patent Document 3: Japanese Translation of PCT International Application No. 2005-531219